This invention relates generally to pollution monitoring systems, and, more particularly to a laser excited pollution monitoring system capable of performing vapor analysis at a plurality of remote locations.
A critical need exists for toxic vapor detection at various locations in, for example, a rocket launch installation. During routine handling operations, at such rocket launch installations, the possibility exists that volatile compounds such as hydrazine, monomethylhydrazine (MMH), and unsymmetrical dimethylhydrazine (UDMH) which are extensively used as rocket fuels can be inadvertently released into the ambient air. The health hazard to humans resulting from exposure to airborne hydrazines, even at very low concentrations, are of increasingly great concern. New standards have been proposed that will reduce the maximum allowable concentrations of hydrazine, MMH, and UDMH in work place ambient air samples, collected over a two hour period, to 30, 40, and 60 PPB, respectively.
The photoacoustic toxic vapor detector is an example of a toxic vapor detector which has been successful in detecting minute quantities of toxic vapors from volatile compounds used as rocket fuel. The detection principle utilized in the photoacoustic vapor detector is based upon the use of an audio-frequency modulated laser, tuned to an absorption frequency of the molecules under test. The laser produces a beam of electromagnetic radiation which irradiates a vessel or cell in which the molecules of the toxic vapor are located. The molecules of the toxic vapor absorb the laser light and reach an excited state. Collisions with the ambient air molecules deactivate the excited molecules thereby causing a periodic pressure rise inside the cell. This periodic pressure rise is in the form of a sound wave capable of being detected by a pressure transducer located within the cell. A more detailed description of such a photoacoustic detector can be found in an article by L. B. Kreuzer et al entitled "Air Pollution: Sensitive Detection of Ten Pollutant Gases by Carbon Monoxide and Carbon Dioxide Lasers," Science, Volume 177, July 28, 1972, pages 347-349.
Unfortunately, the utilization of the photoacoustic detection technique at, for example, rocket launch installation sites, is impractical because of the need of multiple monitoring points throughout the site. This impracticality arises as a result of the requirement of positioning the laser source, photoacoustic head including the cell and signal processor at each monitoring point.
The major portion of the expense of providing a complete photoacoustic detector is the laser. For example, a typical laser capable of being utilized within a photoacoustic detector would be a tunable CO.sub.2 laser. Its cost, however, represents approximately 80% of the total cost of the photoacoustic detector. In addition, the laser is also a major contributor to the size and weight of the unit. Hence, the placement of a complete photoacoustic detector unit at each site results in a monitoring network that is bulky, extremely expensive, difficult to maintain, and as pointed out hereinabove, totally impractical.
It is therefore essential in order to effectively protect personnel at rocket launch sites and the like to provide a toxic vapor detection system which is not only capable of incorporating therein the photoacoustic detection technique, but is also capable of utilizing this technique in an effective and economical manner at a multitude of monitoring sites. In addition, the system should have rapid response, be rugged and highly reliable in operation.